Currently, there is a need to acquire a detailed tomogram of a living body for the purpose of making a diagnosis of cancer or the like. As a method for acquiring a detailed tomogram of a living body, “Time domain OCT” has been proposed from the past, in which a tomographic image of a subject is obtained by scanning with light outputted from a low coherence light source.
Further, in recent years, Frequency domain OCT has been used, which is improved OCT that has solved such problems that an optimal signal/noise ratio (S/N ratio) cannot be obtained, an image capturing frame rate is low, and a penetration depth (an observation depth) is small, which are disadvantages of “Time domain OCT.”
As representative apparatus configurations to carry out Frequency domain OCT measurement, an SD-OCT (Spectral Domain OCT) apparatus and SS-OCT (Swept Source OCT) of two types can be cited.
The SD-OCT apparatus uses broad bandwidth low coherence light such as an SLD (Super Luminescence Diode), an ASE (Amplified Spontaneous Emission) light source, and white light as a light source and splits the broad bandwidth low coherence light into measuring light and reference light using a Michelson type interferometer or the like. Thereafter, the SD-OCT apparatus radiates the measuring light onto a measuring subject, causes reflection light that has been reflected by the measuring subject and has returned and the reference light to interfere with each other, and splits this interfered light into respective frequency components using a spectrometer. Then, the SD-OCT apparatus measures an interfered light intensity for each frequency component using a detector array in which elements such as photodiodes are arranged in an array. The SD-OCT apparatus Fourier-transforms a spectral interference intensity signal that has been obtained by measuring the interfered light intensity with a computer, to thereby compose a tomographic image.
On the other hand, the SS-OCT apparatus uses as a light source a laser that temporally sweeps an optical frequency and causes the reflection light and the reference light to interfere with each other at respective wavelengths. The SS-OCT apparatus measures a temporal waveform of a signal that corresponds to a temporal change in the optical frequency. Then, the SS-OCT apparatus Fourier-transforms a spectral interference intensity signal that has been obtained through the measurement of the temporal waveform with a computer, to thereby compose a tomographic image.
Now, in a diagnosis of cancer through an endoscope, it is typical in recent years that an extent of progression of cancer is determined from a surface condition of a lesion observed through the endoscope.
Thus, in an OCT apparatus (an optical tomographic imaging system) as well, it is extremely effective if not only a conventional tomographic image but also a surface condition of a lesion can be observed at the same time. With such a demand, a technique is disclosed in which a tomogram in a direction parallel to a surface is generated from three dimensional volume data after all tomographic images are acquired and is used in a diagnosis of cancer (see PTL 1).